Relationship between Microstructure and Mechanical Properties in ODS Materials for Nuclear Application

Centre

of

Excellence

for

Nuclear

Materials

Workshop

Materials

Innovation

for

Nuclear Optimized

Systems

December 5-7, 2012, CEA – INSTN Saclay, France

Yann De CARLAN

CEA (France)

Relationship between Microstructure and Mechanical

Properties in ODS Materials for Nuclear Application

Workshop

organized

by:

Christophe

GALLÉ,

CEA/MINOS,

Saclay

–

[email protected]

Workshop

Materials

Innovation

for

Nuclear

Optimized

Systems

December 5-7, 2012, CEA – INSTN Saclay, France

Relationship between Microstructure and Mechanical Properties in ODS

Materials for Nuclear Application

Yann De CARLAN

1

CEA-DEN-DMN, Service de Recherches Métallurgiques Appliquées, SRMA (Saclay, France)

Oxide Dispersion Strengthened ferritic/martensitic alloys are developed as prospective cladding materials for future Sodium-Cooled-Fast-Reactors (GEN IV) [1]. These advanced alloys present a good resistance to irradiation and a high creep rupture strength due to a reinforcement by the homogeneous dispersion of hard nano-sized particles (such as Y2O3 or YTiO). ODS alloys are

elaborated by powder metallurgy, consolidated by hot extrusion and manufactured into cladding tube using the pilger cold-rolling process [2, 3]. ODS alloys present usually low ductility and high hardness. The aim of this talk is to present the specificity of the metallurgy of ODS materials in relationship with the main mechanical properties (tensile and creep properties, toughness, transition temperature…). Two types of alloys will be presented: Fe-9Cr martensitic ODS and Fe-14Cr ferritic ODS alloys. Mechanical properties of the materials depend on the metallurgical state (fine grains, recrystallized, martensitic) and very different behaviors are observed as a function of final microstructure. For example, for a Fe-9CrODS alloy, tempered martensite lets obtaining material with high strength whereassoftenedferriteseefigure1[4] tolerates highdeformation levels.

Tempered martensitic structure

Softened ferritic structure

Fig. 1: Electron Backscatter Diffraction maps for a Fe-9Cr ODS associated with the tensile properties at room temperature.

Reactor fuel pin cladding. Journal of Nuclear Materials 428 (2012) 6–12.

[2] T. Narita, S. Ukai, T. Kaito, S. Ohtsuka, T. Kobayashi, Development of Two-Step Softening Heat Treatment for Manufacturing 12Cr–ODS Ferritic Steel Tubes. Journal of Nuclear Science and Technology, Vol. 41, No. 10, p. 1008–1012 (October 2004).

[3] S. Ohtsuka, S. Ukai, M. Fujiwara, T. Kaito, T. Narita, Improvement of 9Cr-ODS martensitic steel properties by controlling excess oxygen and titanium contents. Journal of Nuclear Materials 329-333(1): 372-376.

MINOS Workshop, Materials Innovation for Nuclear Optimized Systems December 5-7, 2012, CEA – INSTN Saclay, France

AND MECHANICAL PROPERTIES IN ODS

MATERIALS FOR NUCLEAR APPLICATION

Rapsodie

Beginning of the

studies on the Fast

Sodium Reactors to

produce electricity

First ODS

(SCK-CEN)

Phénix

First irradiated

tubes

Superphénix

Industrial

Production

(DourMetal)

ODS tubes in

Phenix reactor

Forum Gen IV : reactors

1967

1973

1985

1987

2000

1960

Materials

Optimization

of the

austenitic

steels

New reinforced

ferritic alloys :

ODS alloys

PHENIX

PHENIX

SPX

SFR

Need of

optimized

materials is still

ODS materials

Limited swelling under irradiation

Low thermal creep

Nano dispersion

(Oxide Dispersion Strengthened)

0 1 2 3 4 5 6 7 8 9 10

60 70 80 90 100 110 120 130 140 150 160 170 180 190

dose (dpa)

Ferritic-martensitic (F/M) steels ODS included

Average 15/15Ti

Best lot of 15/15Ti

Use limit for

Phenix reactor

S

w

e

lli

n

g

(%

)

0,0001 0,001 0,01 0,1 1 10 100 1000 10000 Temps (h) A llo ng em en t ( % )

180 MPa – 650°C

Conventional steel

ODS

SFR

Dose > 150 dpa

Stress ~ 100 MPa

nano-phases

(~2-4nm)

Complex manufacturing

ODS steels present low ductility and limited consolidation at room temperature

Low deformability

Ex:ODS steel

18 Cr-1W Ti 0,5 Y

O

Uniform distribution of

nano oxides

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

D in mm

W t (% ).

0.46 Y

0.3 Ti

0.85 W

Microprobe

Homogeneous microstructure

Fine ferritic grains

elongated

in extrusion direction

Longitudinal direction

Transverse direction

TEM

nano-oxides

TEM

SANS

TAP

Optical

DSM – LLB

Orphée

0 0,2 0,4 0,6

0 1 2 3 4 5 6 7 8 9 10

Rayon moyen (nm)

H

(r

)

*

fp

90×14×14 nm3

Y,YO and O

Clusters

Ti, TiO

8 10

_m

1,8 nm

Texture

8 10

_m

φ

_{3 nm}

1, 2

µ

m - 300nm

400 nm - 200 nm

f

1,1%

GPM Rouen

O. Kalokhtina, B. Radiguet, Ph. Pareige

Plate

0 200 400 600 800 1000 1200 1400

0 1 2 3 4 5 6 7

ODS metallurgy

Conventional metallurgy

Temperature /°C

Ferritic grades

Fe -12/20Cr

Martensitic grades

Fe - 9 Cr

Large differences between both type of alloys as a function of their final

microstructure but generic creep behavior is noticed.

Ferritic Fe-14Cr ODS :

Extruded at 1100°C + 1h

at 1050°C (very difficult to

recrystallize)

Typical microstructures

10µm

Martensitic Fe-9Cr ODS :

Tempered martensite

(very high critical cooling rates)

austenite + ferrite

austenite + martensite

Martensitic grade: Fe-9Cr-1W-0.2Ti-0.3Y

O

CCT diagram

-

Transformation temperatures

-

Critical cooling rates ~ 1°C/s

Tensile properties

Creep properties

Ferritic (Fe-14 Cr ODS)

0 100 200 300 400 500 600

0 1 2 3 4 5 6 7 8 9 10

Allongement rationnel (%)

C

on

tr

ai

nt

e

vr

ai

e

(M

Pa

) 7.10-4 s-1

10-5 s-1

Martensitic (Fe-9Cr ODS)

L. Toualbi, Phd Thesis

Tensile properties in different metallurgical state :

- J95-M3, tempered martensite

- J95- M5, Softened ferrite

Tempered martensite

Ferritic (Fe-18 Cr ODS)

Extrusion direction

ODS plate

Extruded and annealed

Extrusion direction

SL

ST

S45

200 nm

Ferritic Fe-18 Cr ODS at 650°C 250 MPa

M. Ratti, Phd Thesis

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2

0 1000 2000 3000 4000 5000 6000 7000

Temps (heures) Al lo ng em en t ( % )

Very low deformation rates

(Long-250 MPa : 7.10

_h

_).

almost no tertiary domain .

Different order of magnitude

on the rupture time as a function

of the orientation of the sample /

texture of the microstructure

Transvers

45

°

°

°

°

Ferritic ODS

A. Alamo et al. Journal of Nuclear Materials 329–333 (2004) 333–337

0

100

200

300

400

500

10

10

10

10

10

10

10

S

tr

e

s

s

(

M

P

a

)

Rupture Time (h)

MA957 - Fine grains

650°C

9Cr2MoVNb (EM12)

650°C

MA957 - Recryst.

700°C

MA957 - Recryst.

650°C

15/15Ti

700°C

Aust.15/15Ti

650°C

0

100

200

300

400

500

10

10

10

10

10

10

10

S

tr

e

s

s

(

M

P

a

)

Rupture Time (h)

MA957 - Fine grains

650°C

9Cr2MoVNb (EM12)

650°C

MA957 - Recryst.

700°C

MA957 - Recryst.

650°C

19

Martensitic Fe-9 Cr ODS

1

7

10

4

.

1

⋅

−

−

=

s

II

ε

&

1

10

10

1

.

1

⋅

−

−

=

s

II

ε

&

INTERGRANULAR

mechanism

Consistent with the macroscopic observations

ODS

9%Cr

ODS

14%Cr

•

Dislocation still pinned on oxides

•

Viscous motion

•

Activation of sources of

dislocations at the grain boundary

Fine grain

A. Alamo et al. Journal of Nuclear Materials 329–333 (2004) 333–337

DBTT (°C)

USE (J)

LT

TL

LT

TL

rod Ø36

-29

108

7.6

3.8

anisotropic impact behaviour

for the rod: higher USE and

lower DBTT under LT loading

extrusion

direction

TL

LT

diameter: 36mm

Fe-14Cr 1W ODS

“Fine grain

23

AL Rouffié et al. http://dx.doi.org/10.1016/j.jnucmat.2012.08.050

DBTT (°C)

USE (J)

LT

TL

LT

TL

rod Ø36

-29

108

7.6

3.8

anisotropic impact behaviour

for the rod: higher USE and

lower DBTT under LT loading

ED

notch

expected

propagation

plane

effective

propagation plane

For different samples,

tendency to delamination

The mechanical properties of nuclear F/M alloys depend strongly on the chemical composition

and the microstructure of the product

Tensile properties

-

Large dependence on the grain size / morphology / dislocation density / nano-precipitation

-

Collapse of the strength from 400°C

-

Strain rate effect on the ductility

Creep properties

-

No tertiary creep but high performances

-

Reduced fracture strain

-

Damage more severe at high temperature low strain rate / low stress =>

Intergranular

mechanism of failure

Impact properties

-

Low toughness and in general “low” Upper Shelf Energy and sometimes tendency to

delamination

-

Very important impact of the microstructure (morphological texture) on the impact properties